In conventional disk drive electronics, magnetic head circuitry is provided for amplifying signals read using magnetic heads, particularly so-called Magneto-Resistive (MR) heads. However, a common problem occurring during MR head operation is voltage discharge to MR heads arising when the head touches disc media.
Hence, due to sensitivity to possible damage from discharge, known designs for MR amplifiers do not operate using conventional five volt power supply. (See U.S. Pat. Nos. 4,786,993 and 5,327,303). In fact, to reduce such damage, conventional MR amplifier designs require either dual power supply voltages with grounded disk assembly or disk assemblies which float at potential equal to that of MR head. (See U.S. Pat. No. 4,879,610 to Jove et al. wherein negative power supply added to amplifier thereby raising total voltage of circuit over five volts, and rotating disk assembly biased from circuit ground to equalize potential of rotating discs with that of MR head. Additionally, U.S. Pat. No. 5,444,579 to Klein et al., discloses MR amplifier design with one end of MR head grounded, but amplifier bias current cannot be optimized for noise performance since MR head and input transistor share current.)
Furthermore, to operate properly, MR heads are typically DC biased. However, unless such DC bias is removed before read signal is received by amplifier, the amplifier may saturate. Commonly coupling capacitors are used to remove DC component from AC signal. However, value of DC blocking coupling capacitor tend to be relatively large and inappropriate for chip integration, particularly when multiple heads require many such capacitors. (See U.S. Pat. No. 4,833,559 to Belk which multiplexes MR elements into external capacitor, and multiplexing transistors are relatively large, thereby introducing substantial amplifier noise.)
There is a need, therefore, for improved, integrated design for low-noise, magneto-resistive pre-amplifier circuits, particularly for implementation in CMOS technology.